Ultrasonic flow measuring devices are often utilized in the field of process and automation technology. They permit simple determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic flow measuring devices work frequently on the basis of the Doppler principle or on the basis of the travel-time difference principle.
In the case of the travel-time difference principle, the different travel times of ultrasonic pulses in and counter to the flow direction of the liquid are evaluated.
For this, ultrasonic pulses are transmitted at a certain angle to the tube axis both with and counter to the flow. From the travel-time difference, the flow velocity, and, therewith, with knowledge of the diameter of the section of pipeline, the volume flow can be determined.
In the case of the Doppler principle, ultrasonic waves with a certain frequency are coupled into the liquid, and the ultrasonic waves reflected by the liquid are evaluated. From the frequency shift between the coupled-in and reflected Waves, the flow velocity of the liquid likewise can be determined.
Reflections occur in the liquid, however, only when small air bubbles or impurities are present, so that this principle mainly finds application in the case of contaminated liquids.
Ultrasonic waves are produced or received with the help of so-called ultrasonic transducers. For this, ultrasonic transducers are fixedly applied on the pipe wall of the pipeline section of concern. More recently, clamp-on ultrasonic flow measuring systems have also become available. In the case of these systems, the ultrasonic transducers are, essentially, just pressed on the pipe wall with a clamp. Such systems are known e.g. from EP 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
A further ultrasonic flow measuring device, which works according to the travel-time difference principle, is known from U.S. Pat. No. 5,052,230. The travel time is ascertained, in this case, by means of short, ultrasonic pulses.
A large advantage of clamp-on ultrasonic flow measuring systems is that they do not contact the measured medium, and can be placed on an already existing pipeline. Disadvantageous is the greater effort in the mounting of the clamp-on systems, in order to align the individual ultrasonic transducers relative to one another, this depending on many parameters, such as, for example, pipe wall thickness, pipe diameter, and velocity of sound in the measured medium.
Ultrasonic transducers are composed, normally, of an electromechanical transducer—in industrial process measurements technology, most often, a piezoceramic—and a coupling layer, also referred to as “coupling wedge” or, less frequently, “lead-in element”. The coupling layer is, in such case, most often made of synthetic material, or plastic. The ultrasonic waves are produced in the electromechanical transducer element, guided via the coupling layer to the pipe wall, and, from there, conducted into the liquid. Since the sound velocities in liquids and synthetic materials, or plastics, differ, the ultrasonic waves are refracted at the transitions from one medium to another. The angle of refraction is determined, to a first approximation, by Snell's law. In accordance therewith, the angle of refraction depends on the ratio of the propagation velocities in the media.
Between the piezoelectric element and the coupling layer, an additional coupling layer can be arranged, a so-called adapting, or matching, layer. The adapting layer assumes the function, in such case, of transmitting the ultrasonic signal and, simultaneously, reducing reflection caused at the interfaces by different acoustic impedances of the adjoining materials.
In numerous sources, e.g. in DE 10 2006 029 199 B3, the flow velocity of a measured medium in a measuring tube is ascertained via dispersion of an ultrasonic signal by the flow of the measured medium in the measuring tube.
In WO 2007/039394 A2, an ultrasonic flow measurement device is described, wherein the device has at least one ultrasonic transducer in a first region of the measuring tube, and at least two ultrasonic transducers in a second region. Due to the difference in the distances of the transducers in the second region to the transducer in the first region, there results a travel time difference for the ultrasonic signals. This travel time difference is taken into consideration for calculating the flow. A disadvantage is that the ultrasonic transducer in the first region of the measuring tube must produce an energy intensive signal with a high signal strength and broad signal aperture angle in order for the signal to reach the two other ultrasonic transducers in the second region of the measuring tube.
DE 102 21 771 A1 shows an ultrasonic sensor for an ultrasonic flow measurement device having a plurality of piezo elements, which are combined to form a so-called piezo array, wherein the piezo elements are operable in a time-delayed manner. In this way, with an ultrasonic sensor placed flat on the measuring pipe wall, it is possible to obtain different angles of the ultrasonic signal radiated into the measured medium with a wavefront relative to the measuring tube axis. The time-delayed activating is, however, very computationally intensive. Also, the changing of the angle is only feasible in a limited region. If the ultrasonic signal is radiated in a very flat manner, an exciting of longitudinal waves can occur, and the transmission through the pipe wall is lessened, and a significant part of the sound wave is reflected.